1. Field of the Invention
This invention relates to a video signal processing device and more particularly to a video signal processing device having a non-linear emphasis circuit or a non-linear deemphasis circuit.
2. Description of the Related Art
A circuit arranged as shown in FIG. 1 of the accompanying drawings has been proposed as a non-linear emphasis circuit to be applied to a system for transmitting a video signal through a transmission line which has its dynamic range limited because of much noise. Meanwhile a non-linear deemphasis circuit which is arranged as shown in FIG. 2 has also been proposed.
Referring to FIG. 1, an input video signal x is supplied to a high-pass filter (hereinafter referred to as HPF) 41. The high-frequency component of the video signal is separated by the HPF 41. The high-frequency component as separated is amplitude-compressed by a non-linear amplifier (hereinafter referred to as NLA) 42. The NLA 42 has an input-output characteristic as shown in FIG. 3. The input-output characteristic is such that the output amplitude is limited when an input signal has a large amplitude. The output w of the NLA 42 is added by an addition circuit 43 to the input video signal x. As a result, a signal y in which a high-frequency component is emphasized is output from the addition circuit 43. In the case of FIG. 1, the HPF 41 is, for example, composed of a capacitor and a resistor (C and R). The NLA 42 is, for example, composed of a logarithmic compression circuit which uses, for example, a diode.
The frequency characteristic of the output y is a so-called non-linear emphasis characteristic in which the emphasized degree of the high-frequency component varies with the input level as shown in FIG. 4.
The video signal which has its high-frequency component emphasized is fraught with noises as in the case of magnetic recording and reproduction by a VTR. Therefore, the video signal is supplied through a transmission line of a limited dynamic range to a deemphasis circuit which is of a characteristic reverse to that of the above-stated emphasis circuit. The high-frequency component which has been emphasized is suppressed. By this, a noise component added through the transmission line is suppressed, so that an image of high S/N ratio can be obtained. Further, since the degree of emphasis for a high-level signal is small, the dynamic range of the transmission line is never increased for the high-level signal. In that case, therefore, no distortion is brought about by the limitation imposed on the dynamic range of the transmission line.
The deemphasis circuit is arranged as follows: Referring to FIG. 2, an input x' is supplied to a subtracter 53. The output of the subtracter 53 is obtained as a deemphasis output y'. The output y' is supplied to an HPF 41 and an NLA 42 to obtain a high-frequency component w which is amplitude-compressed. The amplitude-compressed high-frequency component is fed back to the subtracter 53. If the characteristic of the HPF 41 and that of the NLA 42 are the same as those of the HPF 41 and the NLA 42 of the emphasis circuit of FIG. 1, the transmission characteristics of the circuits shown in FIGS. 1 and 2 are reverse to each other and the transmission characteristic obtained through the two circuits would completely become "1".
The emphasis and deemphasis circuits of the above-stated characteristics necessitate the use of non-linear amplitude compressors such as logarithmic diode compressors. Generally, however, it has been difficult to obtain a high degree of accuracy, stability and an adequate high-frequency characteristic with the emphasis circuit of such a characteristic. Further, the deemphasis circuit which is arranged in a feedback circuit requires use of a logarithmic compressor having a frequency characteristic which remains stable for a wide band.
Therefore, it has been difficult to use such emphasis and deemphasis circuits for an apparatus that is required to process signals at a high degree of precision over a wide frequency band, such as a VTR of the kind intended to record such a wide-band signal as a high-definition TV signal or the like.
Generally, the apparatus of the kind recording and reproducing color video signals on and from magnetic recording media are contrived in varied manners to be capable of recording and reproducing the video signal for as long a period of time as possible. In one of known methods applicable to a color signal which is not much affected in terms of visual image quality, color-difference signals of two kinds are recorded as a line-sequential color-difference signal by alternately skipping them on each line.
Further, in recording the video signal on a recording medium as mentioned above, the high-frequency component is emphasized or intensified for the purpose of preventing the deterioration thereof. Then, in reproducing the signal, a so-called deemphasizing process is performed in a manner reverse to the emphasizing process. For a color signal, it is also known to perform non-linear emphasis and deemphasis processes for changing the degree of emphasis according to the level of the signal for the purpose of effectively utilizing the dynamic range of the signal.
FIG. 5 shows in outline the arrangement of the conventional magnetic recording and reproducing apparatus which is arranged to perform a line-sequential conversion process on the color-difference signals and also the non-linear emphasis and deemphasis processes. Referring to FIG. 5, terminals 21 and 22 are arranged to receive the color-difference signals of two kinds respectively. The color-difference signals as received are supplied respectively to non-linear emphasis circuits (hereinafter referred to as NLE circuits) 23 and 24. The NLE circuits 23 and 24 are arranged to emphasize the high-frequency components of the color-difference signals at a rate determined according to their levels. Their outputs are supplied to a line-sequential conversion processing circuit (hereinafter referred to as an LSC circuit) 25. The LSC circuit 25 is arranged to alternately produce, as a line-sequential color-difference signal for every line, the color-difference signals which are received through the NLE circuits 23 and 24. The line-sequential color-difference signal is supplied to a magnetic recording/reproducing system 27. For the sake of simplification of illustration, a luminance signal is omitted from FIG. 5. However, the luminance signal is of course arranged to be recorded on a recording medium together with the line-sequential color-difference signal.
The line-sequential color-difference signal which is reproduced by the magnetic recording/reproducing system is supplied to the LSC circuit 29 to be subjected to a process called a simultaneous-conversion process. In other words, a process of forming the color-difference signal of all lines from the color-difference signal obtained from every other line is performed on each of the color-difference signals of the two kinds. The color-difference signals of the two kinds thus obtained are supplied to non-linear deemphasis circuits (hereinafter referred to as NLDE circuits) 31 and 32 respectively. At these circuit 31 and 32, the emphasized high-frequency components are compressed and, after that, are output from output terminals 33 and 34.
With the conventional apparatus arranged as described above, any color-difference signal that has a high frequency and is at a low level is sufficiently emphasized before recording or reproduction. The signal thus can be prevented from being deteriorated by the process of the magnetic recording/reproducing system.
By the above-stated line-sequential conversion process, the sampling frequency in the vertical direction of the image is lowered. It is, therefore, preferable to suppress an aliasing noise by limiting the band of the original signal in the vertical direction of the image to a level below the Nyquist frequency before the line-sequential conversion process.
However, when the LSC circuit 25 of FIG. 5 limits the band of the image signal in the vertical direction, the level of the signal which has undergone the non-linear emphasis process sometimes comes to change at a part of the original image where the vertical resolution is high. As is well known, the degree of emphasis varies to a great degree according to the signal level through the NLE process. Therefore, in a case where the signal level changes to a great degree in the latter stage of the NLE process, the degree of attenuation of the high-frequency component obtained at the NLDE circuit no longer corresponds to the degree of emphasis of the high-frequency component obtained at the NLE circuit. In that case, the original signal cannot be accurately reproduced.